Corrosion resistant high strength alloy

ABSTRACT

A high strength corrosion resistant steel containing correlated amounts of iron, chromium, molybdenum, cobalt, nickel, carbon and other constituents such as aluminum and titanium. The alloys resist pitting and crevice corrosion as well as general corrosion and are capable of offering tensile strengths well above 150,000 psi.

United States Patent [191 Snape [111 3,811,872 [451 May21, 1974 CORROSION RESISTANT HIGH STRENGTH ALLOY [75.] Inventor: Edwin Snape, Monsey, v

[73] Assignee: The International Nickel Company,

Inc., New York, NY.

[22] Filed: Apr. 21,1971

[21] Appi. No.: 136,217

52 us. Cl. 75/122, 75/128 w, 75/128 B,

51 int. Cl. C22c 39/00, C22c 19/00 [58] Field of Sea'i'ch..-..... 75/128 B, 128 R, 122, 171

[56] References Cited UNITED STATES. PATENTS H1951 Clarke 75/125 4/1961 Thielmann... 5/1962 Bourquin 75/1-71 H Primary Examin'er-Hy1and Bizot Attorney, Agent, or Firm-Raymond J Kenny; Ewan C. MacQueen 57 ABSTRACT A high strength corrosion resistant steel containing correlated amounts of iron, chromium, molybdenum, cobalt, nickel, carbon and other. constituents such as aluminum and titanium. The alloys resist pitting and crevice corrosion as well as general corrosion and are capable of offering tensile strengths well above 150,000 psi. H

14 Claims, No Drawings CORROSION RESISTANT HIGH STRENGTH ALLOY As is known in the metallurgical art, there are a num ber of alloys commercially available which might be characterized, relatively speaking, as highly corrosion resistant, many of the stainless steels and certain nickel and copper-base alloys being illustrative. And, there are any number of alloys which upon hardening manifest high tensile strength, the low alloy and maraging steels being rather exemplary. Considerably less But, the demand for morecorrosion resistant materials in such environments is more than likely to persist, as would appear evident from the increased activity in such rapidly developing areas such as desalination, undersea mining, oceanography in general, etc.

When tensile strength requirements on the order of,

say, 150,000 to 275,000 psi together with good fabricability, ductility, etc., are injected into the picture, it becomes more readily. apparent asto why so very few alloys are commercially available capableof operating satisfactorily under such conditions. It is to this overall problem to which the present invention is primarily, though by no means singularly, addressed.

In any case, it has been found that certain hardened alloys containing correlated percentages of iron, nickel, cobalt, chromium, molybdenum, etc., offer exceptional combinations of corrosion resistance and tensile strength together with good fabricability and adequate ductility.

Generally speaking, the present invention contemplates highly corrosion resistant, work hardenable alloys capable of developing a special microstructureand high strength as more fully herein described, the alloys containing (in weight per cent) from 5 to 30 percent chromium, about 2 to 15 percent molybdenum with the total percentage of molybdenum plus chromium being from about 20 to about 35 percent, about to 30 percent nickel, about 13 to 30 percent cobalt, the ratio of nickel to cobalt not exceeding about 1.5:1, up to 0.15

percent, but most beneficially not more than 0.05 or 0.03 percent, carbon, up to'about 3 percent titanium,

up to about 3 percent aluminum and the balance essen-,

tially iron, the iron preferably constituting at least about 15 or percent but not less than 10 percent of the alloys.

In addition to the above chemistry and in order to achieve tensile strength levels as set forth herein, the

' steels should be characterized by a microstructure about 7.5 to 20 percent being quite satisfactory, although up to 45 percent can be present.

In carrying the invention into practice, the sum of the molybdenum plus chromium should not exceed about 35 percent; otherwise, hot working is at best unnecessarily rendered more difficult, due, it is believed, to the tendency -,of these constituents to form sigma, an embrittling phase which can promote the onset ofcracking. For this and other reasons, it is preferred that the total percentage of molybdenum plus chromium not exceed about 30 percent; however, at least 20 percent thereof is required for good corrosion resistance. In striving for the optimum by way of resistance to crevice and pitting corrosion in particular, the chromium should be from 16 to 30 percent, the molybdenum should be from 2 to 10 percent and these components should be correlated such that the total sum of threefourths the percentage of chromium plus the percentage of molybdenum is at least about 18 percent and is beneficially at least about 20 percent.

It is also noteworthy to point out that it has been de- I termined that molybdenum confers improved tensile ductility and also increases the work hardening rate, particularly at reductions of at least 25 percent, e.g., 40 percent, or more. Accordingly, it is preferred that the molybdenum be not less than 4 percent.

With regard to cobalt, it exercises a potent influence in bringing about the formation in the austenite of the epsilon-like platelets by virtue of which the high strength levels characteristic of the subject alloys are in large part achieved. Oddly enough, given the voluminous information available concerning the mechanisms by which alloy matrices are conventionally hardened, e.g., age hardening (including precipitation hardening), cold working, refrigeration, matrix stiffening, combinations thereof, etc., in marked contrast scant attention ostensibly has been accorded the type of strengthening described herein. This might possibly stem from embrittlement considerations. Regardless of the reasons, I am presently aware of' but one instance in which this phenomenon has been advanced commercially. This occurred in respect of a relatively high cobalt-nickel-base alloy, an alloy which appears, at best, to'be difficultly fabricable as well as costly.

Preliminary metallographic examination of various alloys indicatedthe thin parallel plateletsto be of the epsilon phase; however, using electron transmission microscopy and electron diffraction the platelets appeared to be more on the order of deformation twins. It is deemed that this holds true for alloys within the invention up to about 25 percent cobalt. At the higher cobalt percentages, e.g., 28-30 percent, it is considered that epsilon is likely present although it might be difficult to resolve. As is known, deformation twins and the epsilon phase are quite difficult to differentiate. In any case and irrespective of terminology, the thin parallel platelets are formed by cold working alloy compositions within the invention.

The percentage of cobalt need not exceed about 21 or 22 percent since it has been found that higher amounts do not appreciably increase work hardening rate. Thus, it is deemed about 15 percent to 23 or 25 percent cobalt is extremely effective in providing steels at the higher order of tensile strength without adversely affecting other characteristics. Above about 30 percent cobalt, improvement is not sufficiently great to warrant the added cost. Percentages below 13 percent and 3 the ratio of nickel to cobalt does not exceed about 1.2:].

Nickel is particularly useful in precluding formation of unwanted martensite during cold working. It is thought excessive martensite can contribute to or impart a condition of embrittlement. A nickel range of 15 percent to 23 or 25 percent is quite beneficial and a combined nickel plus cobalt content of not greater than 45 or 50 percent is satisfactory from a technological viewpoint and is economically attractive commercially.

Tensile strengths of the alloys can be increased through the incorporation of aluminum and/or titanium. Up to 3 percent each can be present but higher amounts should be avoided to obviate hot working difflculties. A range of 0.05 percent or 0.] to 2 percent of either or both can be used, although a combined percentage of not more than 3 percent ismuch preferred. A small amount of titanium, e.g., from 0.05 percent to 0.3 or 0.5 percent is beneficial for good forgeability and also can provide improved corrosion resistance.

With regard to such elements as silicon, manganese and carbon, while up to 2.5 percent silicon can be added to the subject alloys, particularly in castings, it can detract from corrosion resistance and promote edge cracking during hot working if found in amounts of, say, 1.5 or 2 percent or more in the presence of molybdenum above about 7 or 8 percent or a high combined percentage of chromium plus molybdenum, say, much above 25 or 26 percent, e.g., 28 percent or more.

Such alloys are not recommended and, thus, the per 30 centage of any silicon need not exceed 0.5 or 1 percent. Nor does manganese have to exceed 1 to 1.5 percent and it is preferred that it be below about 0.8 percent. Because of its potential effect on corrosion resistance and carbide formation, carbon should not be found in percentages above about 0.05 percent, a range of from trace amounts to 0.03 or 0.04 percent being preferred.

A particularly advantageous alloy composition in accordance herewith is as follows: about 10 to 28 percent chromium, about 5 to about 10 percent molybdenum, the sum of the chromium plus molybdenum being from 24 to 30 percent, about 15 to 25 percent nickel, about 15 to 25 percent cobalt, carbon in an amount up to 0.05 percent, up to 2 percent titanium, up to 2 percent aluminum, up to l percent silicon, up to 1 percent, e.g.,

ln producing the alloys, the nickel, cobalt,.iron and molybdenum were charged, heated to about 2850F and, for Alloys 6-9, held thereat, for about 5 minutes for a carbon boil, the chromium, silicon and, if any, titanium and aluminum then being added. Alloys l-5 were deoxidized with calcium-silicon and nickelmagnesium. Pouring was conducted at about 2,800F and the resulting ingots (30 lb.) were soaked 2 hours at 2,200F with half of each ingot being rolled at about 2,150F to Az-inch plate, the other half being rolled to /2-inch round bar. Upon solution treating at about 2,200F. for 1 hour, followed by water quenching, the /-inch bars were cold drawn in an amount indicated in Table II to induce formation of the platelets in the austenite. Tensile and elongation properties are also reported in Table II.

TABLE II Alloy c0111 U.T.S., Y.s., EL, No. Reduction,% psi psi As reflected by the data in Table II, tensile strengths well above 200,000 psi were attained together with 40 good ductility. Charpy V-notch results were deterresults are given in Table III.

about 0.2 to 0.8 percent, manganese and the balance TABLE [1] essentially iron, the iron preferably constituting at least 20 or 25 percent of the alloy. Alloy c0111 U.T.S., Y.S.. EL,

In order to give those skilled in the art a better appre- R9duli9n.% p psi 71 ciation of the invention, the following illustrative data 510 3 75 226 900 213 600 9 are given. 203.400 238:300 5 i i '0 m ltin and hi h urit mavacuum 'i p e g p y 4 75 241,500 227,900 9 ter1als (electrolytlc 1ron, cobalt and n1ckel, vacuum 85 257 500 253,300 5 grade chromium, molybdenum pellets, etc.) a serles of 5 5 75 alloys were prepared, the compositions being g1ven 1n 5 85 gig'ggg 15288 3 Table I.

TABLE I C. Si. Mn. Ni. Cr. Co. Mo, T1 Al, Fe. Alloy x1 '71 '71 11 '71 '2 '71 '71 91 3 .020 .33 .50 15.0 11.4 20.4 10.0 11.11. .05 Hal.

11.11. none added. n01 unall ned Although alloys of Table I were vacuum melted, air melting procedures can also be used to marked advantage. In this connection, a I lb. heat (Alloy was produced by air melting practice and thereafter tested in much the same manner as the alloys of Table I. This particular alloy nominally contained 0.03 percent car'- bon', 0.4 percent silicon, 0.4 percent manganese, 25 percent nickel, 20 percent chromium), 20 percent cobalt, 8.5 percent molybdenum, balance essentially iron and usual impurities. Deoxidation was accomplished with calcium-silicon and nickel-magnesium.lt was hot rolled at an initial temperature of about 2,l00F, solution treated at 2,l002,200F and quenched prior to cold rolling. After being cold worked 78 percent and then aged at 900F for 3 hours, the following test results were obtained: an ultimate tensile strength of 260,000 psi, a 0.2 percent yield strength of 250,000 psi, an elongation of 9 percent, a reduction in area of 42 percent, an Rc hardness of 49 and a notched tensile to ultimate tensile ratio of 12:1. Elevated temperature properties were also quite good.

In Table IV are set forth the results of aferric chloride corrosion test which involved exposing cold worked and/or cold worked and aged specimens having a surtions. Also, the alloys can be produced and used in various conventional mill forms including sheet, strip, bar, rod and also as suggested above in the form of high strength wire.

In referring to the iron content of the alloys as constituting the balance or balance essentially, it is to be understood, as will be appreciated by. those skilled in the art, that the presence of other elements is not excluded, such as those commonly present as incidental elements, e.g., deoxidizing and cleansing constituents, and impurities normally associated therewith in small amounts that do not adversely affect the basic characteristics of the alloys. Non essential elements that can be present include up to 2 percent each of copper and manifested a high degree of resistance to 10% H 80 andalso to 10% HClin 7 day tests.

The above corrosion data are indicative of the excellent corrosion behavior characteristic of alloys within the invention. It might also be added that potentiostatic I polarization tests conducted to date confirm that the alloys offer marked resistance to corrosion, the alloys displaying an ability to be readily passivated, particularly at the higher molybdenum levels.

As indicated above, alloys in accordance herewith can be prepared by either vacuum processing, including vacuum refining, or'by air melting techniques. To date, in some instances ductility has been slightly better with vacuum melted materials particularly with cold reductions greater than 90 percent. While air melted alloys have been cold drawn 99 percent in the production of wire, vacuum melted alloys have been successfully cold drawn approximately 99.7 percent while reface area of about 1 square inch in a 10 percent ferric chloride solution for a period of 72 hours. Composifggt gl is fi percent boron and up to tions are also given in Table IV. Crevices were intenp tionally formed by wrapping a i s-inch rubber band Although the present invention has been described in about the speciments. As will be appreciated by those conjunction with preferred embodiments, it is to be unskilled in the art, this overall test is'one of an extremely derstood that modifications and variations may be reaggressive nature. The results given represent the oversorted to without departing from the spirit and scope of all weight loss in milligrams determined with regard to the invention, as those skilled in the art will readily unthe entire specimen surface area. derstand. Such modifications and variations are consid- TABLE IV Wt. Loss, c, Si, Mn, Ni, Cr, c0, Mo, Ti, Cold Alloy Rolled Aged 10 048 I .5l .22 l9.9 20.5 20.3 8.2 .43 nil nil ll .034 .50 .19 25.l 20.5 20.2 8.3 .44 nil nil I2 .024 .50 .27 20.4 20.2 20.1 8.5 .27 nil nil 13* .022 .47 .23 25.4 20.0 20.4 8.5 .25 nil nil air melted; otherwise, vacuum mclled nil less than l mg.

With further regard to the above ferric chloride test, ered to be within the purview and scope of the invennone of the specimens exhibited (upon visual examination and appended claims. tion) any evidence of pitting or crevice corrosion. As I claim: to other corrosive media, alloys within the invention l. A corrosion resistant alloy having high strength and characterized by a micros'tructure comprised essentiallyof a face-centered cubic matrix throughout which. platelets; are substantially and relatively uniformly dispersed, the platelets being present as a result of cold working the alloy and being present at least in a small but effective amount sufficient to impart enhanced tensile strength to the matrix, said alloy consisting essentially of from 5 to 30 percent chromium, from 2 to 15 percent molybdenum with the proviso that the total percentage of molybdenum plus chromium be from about 20 to about 35 percent, about 10 to 30 percent nickel, about ID to 30 percent cobalt with the further provisos that when the cobalt content is less than about 13 percent the ratio of nickel to cobalt does not exceed about 1.2:1 and when the cobalt is 13 percent more the said ratio does not exceed 1.5: 1, up to about 0.15 percent carbon, up to about 3 percent titanium, up to about 3 percent aluminum and the balance essentially iron, the iron constituting at least 10 percent of the alloy.

2. An alloy in accordance with claim 1 containing at least about 13 percent cobalt and at least about 15 percent iron and in which the carbon does not exceed about 0.05 percent and in which the platelets are present in an amount of at least 5 percent by volume.

3. An alloy in accordance with claim 2 in which the platelets are present from about 5 to 45 percent by volume.

4. An alloy in accordance with claim 2 in which the sume of chromium plus molybdenum does not exceed 30 percent.

5. An alloy in accordance with claim 4 in which' iron is present in an amount of at least percent.

6. An alloy in accordance with claim 2 in which the carbon content does not exceed 0.03 percent.

7. An alloy in accordance with claim 2 which contains at least one constituent from the group consisting of titanium and aluminum in anamount of at least 0.05 percent.

8. An alloy in accordancewith claim 7 which contains 0.05 to 3 percent of aluminum plus titanium.

9. An alloy in accordance with claim 2 in which the molybdenum content is at least 4 percent.

10. An alloy in accordance with claim'2 in which the chromium is from 16 to 30 percent, the molybdenum is from 2 to 10 percent and the total sum of threefourths the percentage of chromium plus the percentage of molybdenum is at least 20 percent.

11. An alloy in accordance with claim 2 in which any manganese or silicon does not exceed about 1 percent, respectively.

12. An alloy in accordance with claim 2 consisting essentially of about 10 to 28 percent chromium, 5 to 10 percent molybdenum, about 15 to 25 percent nickel, about 15 to 23 percent cobalt, about 0.001 to 0.05 percent carbon, up to 2 percent titanium, up to 2 percent aluminum, up to 1 percent silicon, and up to about 0.8 percent manganese.

13. An alloy in accordance with claim 12 containing at least 0.2 percent manganese.

14. A ductile, workable, corrosion resistant alloy possessing a high degree of resistance to oxidizing chloride media such as stagnant seawater and adapted to afford high tensile strength and consisting essentially of from 5 to 30 percent chromium, about 2 to 15 percent molybdenum with the proviso that the total percentage of molybdenum plus chromium be from about 20 to about 30 percent, about 10 to 25 percent nickel, about 15 to 23 percent cobalt, with the further proviso that the ratio of nickel to cobalt does not exceed 1521, up to about 0.15 percent carbon, up to about 3 percent titanium,.up to about 3 percent aluminum and the balance essentially iron, the iron constituting at least 20 percent of the alloy. 

2. An alloy in accordance with claim 1 containing at least about 13 percent cobalt and at least about 15 percent iron and in which the carbon does not exceed about 0.05 percent and in which the platelets are present in an amount of at least 5 percent by volume.
 3. An alloy in accordance with claim 2 in which the platelets are present from about 5 to 45 percent by volume.
 4. An alloy in accordance with claim 2 in which the sume of chromium plus molybdenum does not exceed 30 percent.
 5. An alloy in accordance with claim 4 in which iron is present in an amount of at least 20 percent.
 6. An alloy in accordance with claim 2 in which the carbon content does not exceed 0.03 percent.
 7. An alloy in accordance with claim 2 which contains at least one constituent from the group consisting of titanium and aluminum in an amount of at least 0.05 percent.
 8. An alloy in accordance with claim 7 which contains 0.05 to 3 percent of aluminum plus titanium.
 9. An alloy in accordance with claim 2 in which the molybdenum content is at least 4 percent.
 10. An alloy in accordance with claim 2 in which the chromium is from 16 to 30 percent, the molybdenum is from 2 to 10 percent and the total sum of three-fourths the percentage of chromium plus the percentage of molybdenum is at least 20 percent.
 11. An alloy in accordance with claim 2 in which any manganese or silicon does not exceed about 1 percent, respectively.
 12. An alloy in accordance with claim 2 consisting essentially of about 10 to 28 percent chromium, 5 to 10 percent molybdenum, about 15 to 25 percent nickel, about 15 to 23 percent cobalt, about 0.001 to 0.05 percent carbon, up to 2 percent titanium, up to 2 percent aluminum, up to 1 percent silicon, and up to about 0.8 percent manganese.
 13. An alloy in accordance with claim 12 containing at least 0.2 percent manganese.
 14. A ductile, workable, corrosion resistant alloy possessing a high degree of resistance to oxidizing chloride media such as stagnant seawater and adapted to afford high tensile strength and consisting essentially of from 5 to 30 percent chromium, about 2 to 15 percent molybdenum with the proviso that the total percentage of molybdenum plus chromium be from about 20 to about 30 percent, about 10 to 25 percent nickel, about 15 to 23 percent cobalt, with the further proviso that the ratio of nickel to cobalt does not exceed 1.5:1, up to about 0.15 percent carbon, up to about 3 percent titanium, up to about 3 percent aluminum and the balance essentially iron, the iron constituting at least 20 percent of the alloy. 